What is the speed of gravity? (Intermediate)

I've been thinking about gravity and would like to ask if we actually recorded the speed in which it moves.

In the theory of relativity, the speed of gravity should be equal to the speed of light, since the theoretical "particles" that carry gravity (sometimes called gravitons) are massless particles, just like photons (the particles that carry light). The light from the Sun takes 8 minutes to reach the Earth, so that if the Sun suddenly disappeared it would take 8 minutes before it got dark. Similarly the Earth would also feel the effects of the Sun's gravity for 8 minutes after it magically vanished.

In September 2002, two US scientists made some very accurate measurements of the position of a quasar as it passed behind Jupiter. They argued that the exact amount of apparent motion of the quasar (as the path of the radio waves from it was bent in Jupiter's gravitational field) depended on both the speed of light AND the speed of gravity. The measurements they took then proved that the speed of gravity is the same as that of light, ruling out some of the more bizarre modifications to the laws of gravity which have been proposed, and further backing General Relativity (BBC news article on the experiment).

More recently, LIGO's first detection of gravitational waves coming from a binary neutron star set an much more accurate limit for the difference between the speed of light and the speed of gravity: the difference is just 10-16 times the speed of light. This bound set by LIGO's detection makes us feel confident that the speed of gravity is, for almost all practical purposes, equal to the speed of light.

About the Author

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.